Earth’s Atmosphere Horizontal Distribution of Air Pressure:
The distribution of atmospheric pressure over the globe is known as the horizontal distribution of pressure.
It is shown on maps with the help of isobars.
An isobar is a line connecting points that have equal values of pressure.
The spacing of isobars expresses the rate and direction of change in air pressure.
This change in air pressure is referred to as the pressure gradient.
The pressure gradient is the ratio between pressure difference and the actual horizontal distance between two points.
Close spacing of isobars expresses a steep pressure gradient while wide spacing indicates a gentle pressure gradient.
The horizontal distribution of atmospheric pressure is not uniform in the world.
It varies from time to time at a given place; it varies from place to place over short distances.
The factors responsible for variation in the horizontal distribution of pressure are as follows:
 Air Temperature
Generally, there is an inverse relationship between air temperature and air pressure.
The higher the air temperature, the lower is the air pressure.
Air pressure is low in equatorial regions and it is higher in Polar Regions.
Along the equator lies a belt of low pressure known as the “equatorial low or doldrums”.
Low air pressure in equatorial regions is due to the fact that hot air ascends there with a gradual decrease in temperature causing thinness of air on the surface.
In the polar region, cold air is very dense hence it descends and pressure increases.
 The Earth’s Rotation
The earth’s rotation generates centrifugal force.
This results in the deflection of air from its original place, causing a decrease of pressure.
It is believed that the low-pressure belts of the sub-Polar Regions and the high-pressure belts of the sub-tropical regions are created as a result of the earth’s rotation.
The earth’s rotation also causes convergence and divergence of moving air.
Areas of convergence experience low pressure while those of divergence have high pressure.
 Pressure of Water Vapour
Air with a higher quantity of water vapour has lower pressure and that with lower quantity of water vapour has higher pressure.
In winter the continents are relatively cool and tend to develop high-pressure centers; in summer they stay warmer than the oceans and tend to be dominated by low pressure, conversely, the oceans are associated with low pressure in winter and high pressure in summer.
Click the following links to Read more:
- The Equatorial Low Pressure Belt
- The Sub-tropical High Pressure Belts
- The Sub-polar low Pressure Belts
- Winds-Pressure Gradient and the Coriolis Effect